In manufacturing semiconductor elements, there has been applied a lithographic technology of forming a photoresist film on a substrate such as a silicon wafer, selectively irradiating the photoresist film with active light rays, and subjecting it to development processing to form thereby a resist pattern on the substrate.
In recent years, in order to obtain a higher degree of integration in LSI, reduction in the line width of worked wires in the lithography process has rapidly been promoted. In this promotion of reduction in the line width of worked wires, various proposals have been made with respect to all steps and all materials to be used for the lithography including photoresists, anti-reflective coatings, exposing methods, exposing apparatuses, developing agents, developing methods and developing apparatuses. For example, various proposals have been made as to the anti-reflective coating. JP-A-60-38821, JP-A-62-62520, JP-A-62-62521, JP-A-5-74700, JP-A-5-188598, JP-A-6-118630, JP-A-6-148896, JP-A-8-305032, JP-A-9-50129, JP-A-9-90615 and JP-A-11-124531 describe to provide a surface reflection-preventing layer having a low refractive index on a resist layer to thereby prevent reflection at the resist surface and detrimental influences by a reflected light on formation of a resist pattern. Formation of the anti-reflective coating on the resist layer serves to reduce fluctuation in sensitivity and therefore reduce dimensional fluctuation even when thickness of the resist layer fluctuates, because amplitude width in a resist film thickness vs. sensitivity curve is made smaller. In addition, use of the surface reflection-preventing film provides another advantage that unfavorable standing wave to be formed due to interference between an incident light and a reflected light or between reflected lights can be reduced.
Further, as to exposing methods, there have been proposed methods of using a light source emitting a light of short wavelength effective for forming a finer pattern, that is, methods of using a deep-UV rays such as KrF excimer laser (248 nm) or an ArF excimer laser (193 nm), or X rays or electron beams, as a light source for exposure, with part of them being put into practice. In the lithography process using such light source emitting the light of short wavelength, there have been proposed chemically amplified resists showing a high sensitivity for the energy rays of a short wavelength as disclosed in JP-A-2-209977, JP-A-2-19847, JP-A-3-206458, JP-A-4-211258 and JP-A-5-249682.
By the way, in order to obtain excellent anti-reflective properties, it is generally believed to be necessary to satisfy the conditions of the following formula 1 and formula 2:ntarc=√{square root over (nresist)}  (1)wherein ntarc represents a refractive index of an anti-reflective coating, and nresist represents a refractive index of a resist;dtarc=x·λ/4ntarc   (2)wherein dtarc represents a thickness of the anti-reflective coating, λ represents a wavelength of energy rays, and x represents an odd integer.
As is apparent from these formulae, the anti-reflective ability is decided by the refractive index of an anti-reflective coating for the wavelength of a light emitted from an intended light source and the thickness of the anti-reflective coating and by the refractive index of a resist. The refractive index of the anti-reflective film is required to be at a low level in comparison with the refractive index of the resist so as to satisfy the above-described conditions.
On the other hand, compounds containing a fluorine atom show a low refractive index due to the characteristics of fluorine atom that fluorine atom has a large molecular volume and a small atomic refractive index, and the refractive index value of such compound is known to be almost proportional to the fluorine content of the compound. Therefore, fluorine-containing compounds are preferred compounds in view of attaining anti-reflecting properties. It is also known to use a fluorine-containing polymer as the fluorine-containing compound having a low refractive index for a material for forming an anti-reflecting coating of a resist layer. In addition to having a low refractive index, the material for the anti-reflecting coating is required to have various properties such as coating properties, film-forming properties and developability with an aqueous solvent. For example, the above-mentioned JP-A-9-50129 and JP-A-11-124531 describes to provide, on a resist layer, a surface reflection-preventing layer containing a fluorine-containing polymer which is soluble in an alkaline aqueous solution and which has a low refractive index to prevent thereby detrimental influences of a reflected light from the resist surface on formation of the resist pattern and, upon developing with an alkaline developing solution, remove the anti-reflective coating together with the resist film. On the other hand, in the case where the above-mentioned fluorine-containing polymer is absent, the refractive index of the film increases and, thus, the standing wave effect and the multi-reflection effect are not sufficiently depressed and, as a result, dimensional accuracy of the resist is deteriorated.
However, in the case of applying, for example, a composition for an anti-reflective coating on a chemically amplified photoresist film, round-topped or T-topped resist patterns which cause troubles in the etching process might possibly be formed with an ill-matched combination of the composition and the chemically amplified photoresist. For example, the above-mentioned JP-A-11-124531 discloses a coating composition containing a fluorine-containing polymer and having a suitably low refractive index as an anti-reflective coating, as an anti-reflective coating composition to be applied on a photoresist film. However, in the case of using this anti-reflective coating composition in a process of using as an exposing light source a short-wavelength light source such as a KrF excimer laser (248 nm) and using a chemically amplified photoresist as a photoresist, there tends to result a developed resist pattern having a T-shaped cross section (T-top) in the case where the photoresist is positive-working.
Further, in recent years, PFAS (perfluoroalkyl sulfonates) having conventionally been used in a composition for anti-reflective coatings has become a subject of discussion with respect to its safety in view of accumulation in human body. For example, there is a movement of providing restrictions on import and production thereof by EPA (Environment Protection Agency) of USA through SNUR (Significant New Use Rule, dated Mar. 11, 2002). Under such circumstances, a PFAS-free composition for anti-reflective coating is being eagerly demanded.
With the above-mentioned background in mind, it is intended in the present invention to provide a method for forming a resist pattern not suffering deterioration of the pattern form such as T-tops and round tops disadvantageous in the etching step to be caused by, for example, intermixing of the chemically amplified resist and the anti-reflective film by forming an anti-reflective film which functions as an interference-preventing film for preventing deterioration of dimensional accuracy of a pattern (variation of dimensional width of the pattern) to be caused within the photoresist film due to interference with a reflected light from the substrate, and to provide a PFAS-free composition for the anti-reflective coating to be used in this method.
As a result of intensive investigations, the inventors have found that the above-described object can be attained by applying an anti-reflective coating composition containing a fluorine-containing polymer on, for example, a chemically amplified photoresist film formed on a substrate to make the surface hydrophilic, and then exposing and developing the photoresist to obtain a resist pattern, with pH of the anti-reflective coating composition being adjusted to 7 or less, thus having completed the invention.